By John Whitesides

WASHINGTON (Reuters) - Democratic leaders in the U.S. House of Representatives scrambled on Friday to allay lingering concerns about a broad healthcare overhaul and said a landmark vote planned for Saturday could slip a day or more.

President Barack Obama and top administration officials called undecided Democrats to plead for support, and House leaders held talks with wavering members to nail down their backing.

Democrats were short of the 218 votes they need to pass the measure, but House Democratic leader Steny Hoyer said "we're very close." House members were warned the final vote could slip to Sunday or even later in the week.

Democrats cajoled dozens of party moderates concerned about abortion and immigration provisions in the bill, as well as its $1 trillion price tag and its possible effect on budget deficits.

"There are many people who are still looking to get a comfort level that this is the right thing to do," Hoyer said. "We're trying to answer any concerns they might have."

The sweeping overhaul, Obama's top domestic priority, would spark the biggest changes in the U.S. healthcare system since the creation of the Medicare health program for the elderly in 1965.

House Republicans are united in opposition to the measure, designed to rein in costs, expand coverage to millions of uninsured and bar insurance practices such as denying coverage to people with pre-existing conditions.

Republicans object to new taxes to pay for the changes and the potential impact on the budget, and say the government is meddling in private healthcare and insurance markets.

LATE-NIGHT NEGOTIATIONS

Democrats on both sides of the abortion issue negotiated late into the night in House Speaker Nancy Pelosi's office, considering proposals to strengthen the bill's provisions to ensure federal subsidies are not used to pay for abortions.

About 40 moderate House Democrats have threatened to oppose the bill without the changes, but members who favor abortion rights said they would not allow the bill to exceed current restrictions on using federal money to finance abortions.

Some negotiators said no deals had been struck but they were still hoping for progress.

"I'm feeling more confident we'll have the votes to pass it," House Energy and Commerce Committee Chairman Henry Waxman said of the bill. "Members who looked like they weren't going to be for the bill are coming around."

If the bill passes the House, the action would move to the Senate, which is preparing its own version. Obama wants to sign a bill by year's end, but Senate Democratic leader Harry Reid has indicated that deadline may not be met.

Obama is expected to visit Congress on Saturday to rally Democrats to support the House bill, which was bolstered on Thursday with endorsements from powerful lobbying groups for doctors and older Americans.

Failure in the Democratic-controlled House would be a huge political blow to Obama. Democrats can afford to lose 40 of their 258 House members on the healthcare vote, but the ranks of potential defectors is even larger.

Democratic Representative Jason Altmire, still undecided, said he got calls from Obama, White House Chief of Staff Rahm Emanuel, Health Secretary Kathleen Sebelius and Education Secretary Arne Duncan, and spoke with Pelosi and Hoyer.

"They have the whole town out lobbying right now," he said. "There is definitely a sense of urgency that you can feel."

Democratic Representative Frank Kratovil said he would vote against the bill because it does not do enough to curb growth in costs and he is concerned about its effect on small businesses.

He said he had not heard from Obama or Pelosi since his decision on Thursday to oppose the bill but had been talking to plenty of other House members.

NO POINT IN LOBBYING

"At this time there is not much point in lobbying me," he said. "I am a 'no.'"

House Democrats plan to open debate on Saturday morning and Hoyer said he expected a final vote on Saturday night. But he warned Republicans might delay it, and he said the House would keep working through Sunday or later if needed.

The House bill would require individuals to buy insurance and all but the smallest employers to offer health coverage to workers. It would subsidize insurance purchases and eliminate the industry's exemption from federal antitrust laws.

Congressional budget analysts say the bill would extend coverage to 36 million uninsured people living in the United States and would reduce the budget deficit by about $100 billion over 10 years.

In an effort to resolve the abortion dispute, Democratic leaders said they would incorporate an amendment by Representative Brad Ellsworth, an opponent of abortion rights, into the rule setting the terms of the debate.

That amendment would guarantee access to insurance plans that agree to refrain from covering abortion. It has been attacked as unsatisfactory by other anti-abortion Democrats and groups like the National Right to Life Committee.

(Editing by Vicki Allen, Todd Eastham and Paul Simao)